The present invention pertains generally to lasers and more specifically to HF chemical lasers.
The first continuous wave (cw) hydrogen fluoride (HF) chemical laser was demonstrated by Spencer et al, as reported by D. J. Spencer, T. A. Jacobs, H. Mirals, and R. W. F. Gross, Int. J. Chem. Kinetics, (1969) Vol. 1, pg. 493. Prior to that work, various pulsed or quasi-cw chemical lasers were observed, dating back to the first chemical laser, i.e., the pulsed HCl chemical laser in 1965, as reported by J. V. V. Kasper and G. C. Pimentel, Appl. Phys. Lett., (1964) Vol. 5, pg. 231, and R. W. F. Gross and J. F. Bott, Handbook of Chemical Lasers, (New York, 1976). These articles are specifically incorporated herein by reference for all that they disclose.
Both HF and DF chemical lasers have proven to be of particular interest to the laser community since they can be chemically driven and are scalable to very high output powers for both cw and pulsed operation. Chemically driven lasers principally derive energy from a chemical reaction to populate excited states and, consequently, do not require high input powders from low efficiency mechanisms.
HF lasers, to date, operate on fundamental transitions which constitute vibrational population inversions over a single vibrational transition (.DELTA.v=1). These transitions primarily occur from the v=2 to v=1 vibrational levels and v=1 to v=0 vibrational levels. These fundamental transitions provide scalable output powers in a chemically driven laser which produces extremely useful ouput frequencies in the near infrared spectral region. Continuous wave (cw) HF lasers are commonly available for laboratory use which produce output powers of up to 200 watts in the 2.8 to 3.8 micron regions. Both combustion driven cw HF lasers and electrical discharge pulsed HF lasers are capable of producing extremely high output powers on the fundamental transitions, i.e., .DELTA.v=1.
Although these fundamental transitions produce wavelengths in the 2.8 and 3.8 micron region which are extremely useful in a number of applications, high-power lasers having shorter wavelengths, e.g., in the near ir and spectral region, would also have great utility. For example, various transmission windows exist for atmospheric transmission in the near ir and visible spectral region. Consequently, a laser which could produce a high-power output in the near ir spectral region could be efficiently propagated through the atmosphere for purposes such as power transmission, communications, and various other functions.
High-power laser sources in the near ir region have not only been the subject of intensive research in the past few years because of the atmospheric transmission characteristics of near ir radiation, but for other reasons such as power coupling efficiencies of this wavelength radiation with solid materials. For example, a significant amount of research has been done regarding the power coupling efficiencies of shorter wavelength laser radiation to laser fusion pellets in order to produce a controlled thermonuclear reaction. Power coupling efficiencies of shorter wavelength radiation are also of interest in materials testing experimentation.
Moreover, transmission of laser radiation over standard fiberoptic (silica) materials is much more efficient at near ir frequencies than at longer wavelengths. At near ir frequencies, several hundred watts can be transmitted via conventional fiberoptic materials over long distances, i.e., on the order of miles. Transmission of high powers via fiberoptics would allow conventional silica fiberoptic materials to be used in laser surgery. Currently, special fiberoptic materials must be used to transmit laser radiation at high powers for the frequencies produced by currently available lasers. These materials have been found to be expensive and toxic to the human body, requiring the use of additional expensive coatings of the toxic fiberoptic material. These disadvantages could be overcome by the production of a high-power laser source in the near ir spectral region which could be transmitted through inexpensive, non-toxic silica fiber-optic materials.
Consequently, there has been a significant effort by both government and private industry to develop a high-power laser in the visible and near ir spectral region. To date, however, a scalable high power laser has not been produced which lases in the near ir or visible spectral region despite the intensive effort by both the government and private industry to develop such a laser.
In an attempt to develop such a laser, various techniques have been used to achieve a wide range of laser output frequencies from laser media, such as molecular gases, which are capable of producing scalable high power lasers. A selection of various vibrational and rotational transitions of molecular gases results in a large number of possible spectral lines which can be produced by a particular molecular gas. For example, 33 different vibrational transitions and 37 different pure rotational transitions for the HF molecule are listed in Introduction to Gas Lasers: Population Inversion Mechanisms, Colin S. Willett, (Pergamon Press: New York, N.Y., 1974), which is specifically incorporated herein by reference for all that it discloses. Studies of vibrational transitions of molecular gases have indicated that certain vibrational transitions known as fundamental transitions are much more likely to occur than other transitions. For example, overtone transitions have been reported by both F. G. Sadie, P. A. Buerger, and O.G. Malan in Journal of Appl. Phys., (1972), Vol. 43, No. 6, p. 2906, and Steven N. Suchard and George C. Pimentel in Applied Phys. Lett., (1971) Vol. 18, p. 530 for a continuous wave overtone CO chemical laser and a pulsed deuterium fluoride (DF) vibrational overtone chemical laser, respectively. Both of these articles are specifically incorporated herein by reference for all that they disclose. As both of these articles disclose, and as commonly believed in the industry, overtone transitions do not produce output powers or efficiencies which approach the output powers obtainable from fundamental transitions. As specifically disclosed in the Suchard and Pimentel article, threshold levels were barely able to be reached using overtone transitions in DF molecular gas. Additionally, Suchard et al and Sadie et al both utilized photo-dissociation techniques which are an extremely inefficient process for producing excited vibrational states which cannot practically be scaled to higher output powers.
Another technique of shifting the spectral output of molecular gaseous lasers is the use of various isotopes of the elements of the molecular gas to obtain shifted vibrational levels. For example, a wavelength shift can be obtained through the use of deuterium fluoride (DF) molecular gases instead of HF molecular gases to obtain a wavelength shift of the spectral lines. However, since deuterium fluoride is a heavier molecule than hydrogen fluoride, longer wavelength radiation is produced by DF, rather than shorter wavelength radiation. Consequently, the prior art has indicated that production of shorter wavelength radiation using standard techniques of frequency shifting has not been obtainable in a laser which is scalable to high powers.